The present invention relates to a method for producing a molded body of a powder of a ceramic and/or a metal, and more particularly a method for producing a powder-molded body obtained by casting a gelling agent-containing slurry and solidifying the slurry by gelling the slurry (so-called a gel-cast molding method).
Conventionally, methods such as press molding, cast molding, an injection molding, and the like have been employed as methods for producing a molded body of a powder of a ceramic and a metal and these methods respectively have problems that it is difficult to form a product with a complicated shape, that the size precision is low, that cracking easily takes place in the case of a thick product, that the density of a molded body is hardly increased, and the like and thus the methods are not necessarily satisfactory.
Therefore, attention has recently been paid to a gel-cast molding method (hereinafter referred simply as to a gel-cast method) as a method for solving these problems.
The gel-cast method is a production method of a powder-molded body for obtaining a molded body by casting a slurry containing a powder of a ceramic and/or a metal, a dispersion medium, and a gelling agent and solidifying the slurry by gelling the slurry by controlling the temperature conditions, adding a cross-linking agent, and the like. The gel-cast method is characterized in that a product with a complicated shape can easily be formed since a slurry can be poured in a form in a highly fluid state before gelling and additionally characterized in that a molded body with a sufficiently high strength to stand handling can be obtained by gelling the slurry after casting.
Practically, a method comprising the steps of producing a slurry by dispersing a prepolymer as a gelling agent such as a poly(vinyl alcohol), an epoxy resin, a phenol resin, and the like together with a powder of a ceramic or a metal in a dispersion medium, casting the slurry, and then gelling the slurry by three-dimensionally cross-linking the gelling agent by a cross-linking agent and the like are employed for solidification of a slurry.
However, the above described conventional gel-cast method has the following problems.
At first, the workability of the casting is not necessarily satisfactory to the present requirement and there exist serious problems that cracks are formed at the time of drying the molded body and that significant deformation takes place following insufficient sintering and increase of shrinkage percentage at the time of sintering the molded body.
That is, in the case of a conventional method, because the prepolymer exists in a non-reactive dispersion medium while being diluted and therefore the prepolymer is required to be added in a large amount together with a large amount of cross-linking agent in order to be cross-linked, which inevitably increases the viscosity of the slurry, or because the slurry contains large amounts of the non-reactive dispersion medium, the prepolymer, and the cross-linking agent and it results in existence of large amounts of components, other than the powder, to be evaporated and fired out by drying and firing in the molded body to be subjected to drying and firing.
Secondary, there exists a problem that cracks are formed attributed to unevenness of the shrinkage percentage in respective parts of a molded body at the time of drying and firing.
That is because it is difficult to evenly disperse both of the gelling agent and the cross-linking agent in a slurry with a high concentration and therefore the entire slurry cannot evenly be solidified to result in uneven hardness.
Thirdly, in the case of high viscosity of the prepolymer itself, there exists a problem that since a highly fluidic slurry cannot be produced, the density of the molded body is lowered.
Such a decrease of the density of the molded body results in increase of drying shrinkage and firing shrinkage to make it easy to cause cracks at the time of drying and firing the molded body and consequently leads to decrease of the shape precision of a sintered body.
On the other hand, although a method of using a prepolymer with a low molecular weight and a low viscosity can be considered there exists a problem that a slurry cannot sufficiently be solidified in the case of using a prepolymer with a low molecular weight.
The present invention has been achieved while these problems in conventional techniques being taken into consideration, and an object of the present invention is to provide a production method of powder-molded body capable of improving the gelling (solidification) efficiency and obtaining a molded body with a high density, a low firing shrinkage, and a high shape precision while scarcely causing cracks at the time of drying and firing.
The inventors of the present invention have found that the above described problems on the conventional techniques can be solved by solidifying the gelling agent with the dispersion medium but not with a cross-linking agent by chemically bonding them and have achieved the present invention.
The present invention provides a method for producing a powder-molded body, comprising: casting a slurry containing a powder of a ceramic and/or a metal, a dispersion medium, and a gelling agent; and solidifying the slurry by gelling the slurry to obtain a molded body, wherein the slurry is solidified by chemically bonding an organic dispersion medium having reactive functional groups and the gelling agent.
In the production method of the present invention, it is preferable to use an organic dispersion medium having two or more reactive functional groups and it is preferable to use an organic dispersion medium having reactive functional groups in 60% by mass or more in the total dispersion medium.
In the production method of the present invention, it is preferable for the organic dispersion medium having reactive functional groups to have viscosity of 20 cps or lower at 20xc2x0 C. and it is preferable for the gelling agent to have viscosity of 3,000 cps or lower at 20xc2x0 C. More particularly, it is preferable to solidify a slurry containing an organic dispersion medium having two ore more ester bonds and a gelling agent having isocyanate group and/or isothiocyanate group by chemically bonding the organic dispersion medium and the gelling agent. Further in this case, it is preferable that MDI (4,4xe2x80x2-diphenylmethane diisocyanate) type isocyanate or HDI (hexamethylene diisocyanate) type isocyanate respectively having a basic chemical structure defined as the following chemical formula (1) or (2) is used as the gelling agent and MDI (4,4xe2x80x2-diphenylmethane diisocyanate) type isocyanate having a basic chemical structure defined as the following chemical formula (1) is furthermore preferable to be used as the gelling agent. 